1. Field of the invention
The present invention relates generally to devices and methods for treating spinal disorders and more specifically to a system and method for securing in place an intervertebral device for aligning and maintaining the relative position of adjacent vertebrae to facilitate immobilization of the vertebra through fusion while avoiding stress shielding.
2. Description of the Background
Degeneration of the intervertebral discs and the concomitant instability and translocation of the vertebra is a common cause of back pain and may result from a variety of problems including congenital deformity, age related degeneration, osteoporosis, tumor and disc herniation as a result of trauma. Disc degeneration, for whatever reason, results in compression of the spinal nerve roots resulting in pain. Palliative care is often successful in mild cases but more extreme or degenerative cases may require a surgical approach to stabilize the joint and relieve pressure.
A number of surgical approaches have been developed with varying degrees of success depending on the cause and severity of the damage. A ruptured disc impinging the nerve root may be partially excised to relieve pressure. In such a case the adjacent vertebra may be further fixated using rods, screws and plates in an attempt to stabilize the spine and delay or prevent further degeneration. Patients undergoing such excisions and fixations however often require subsequent procedures to address recurrent pain. In many case such subsequent procedures include fusion of the adjacent vertebra. Spinal fusion, or spondylosyndesis, is a surgical technique to combine two or more vertebrae utilizing supplementary bone graft tissue in conjunction with the body's natural osteoblastic processes to eliminate relative movement as a source of pain. A variety of approaches to fusion are available including posterior fusion, postero-lateral fusion and anterior or posterior interbody fusion.
In the more traditional posterior fusion approach, growth is induced between the bony vertebral laminae to fix the position of the vertebra. In the postero-lateral fusion method bone growth is induced to join the transverse processes to prevent motion between the adjacent vertebrae. However, both posterior and postero-lateral fusion tends to cause bony overgrowth leading to nerve root compression and pain by spinal stenosis. This, coupled with other risks, limitations and disappointing fusion success rates have caused surgeons searching for alternate fusion means to develop interbody fusion techniques.
Interbody fusion techniques involve complete excision of the soft disc which is then replaced with autograft material harvested from the patient, prepared allograft from a donor source or, more recently, synthetic graft material and bone morphogenic protein. Most commonly performed in the lumbar region, the procedure can be accomplished from an anterior approach (Anterior Lumbar Interbody Fusion or ALIF) or a posterior approach (PLIF). In either case the procedure attempts to reconstruct the normal anatomic relationships between the bony and the neural structures and has many advantages. Specifically, weight bearing through a solid bony fusion mass between vertebral bodies relieves the mechanical pain of the traditional unstable degenerative disc and generally prevents long term disc collapse or further degenerative changes. The complete disc excision prevents recurrent herniation of the same degenerated disc.
Successful fusion results in a contiguous growth of bone to create a solid mass that will unite the vertebra into one unit. When fusion graft material is first placed in the intervertebral space it is soft and lacking in cohesive strength so as to be incapable of remaining in position or carrying any load without assistance. A variety of appliances have been developed that attempt to hold the vertebrae to be joined in place relative to one another under normal spinal activity and daily stress to allow the fusion process to occur over the 18-24 month period generally required. Such appliances are often referred to as interbody cages and provide a mechanically rigid scaffold in which the graft material may be placed.
Cage designs vary widely but generally fall into one of three categories. Horizontal cylinders are generally made from titanium and inserted by either the posterior or anterior approach into complimentary holes bored into the intervertebral space. They can be placed by open or minimally invasive techniques. U.S. Pat. No. 5,026,373 to Ray, et al. discloses a cage of this design that includes a perforated threaded exterior surface that can be screwed into place between the vertebra and packed with bone material. Bone growth through the perforations and into the cancellous bone of the vertebra exposed by the insertion results in the desired fusion.
A second design in the form of a vertical cylinder or ring is often referred to as a Harms cage and is also typically made from titanium. The Harms cage can be cut to length as desired so as to span larger segments of the lumbar spine. End caps are employed to prevent subsidence into the cancellous bone although this design suffers, as a result, from a requirement that its central void be pack with graft material prior to insertion. Due to its sharp edges the Harms cage is most commonly inserted by open techniques. U.S. Pat. No. 5,989,290 to Biedermann et al, et al. discloses a cage of this design.
A third design form is the open box cage. Typically constructed of carbon, titanium or bio-compatible non-metallic materials such as PEEK (Polyether ether ketone) or Delrin® (Polyoxymethylene), this design can be formed for an anatomical fit or to recreate the normal lumbar lordosis. Openings in the box walls permit graft material contained within to contact the vertebral bone. Some designs utilize a single large cage inserted by anterior approach. Alternately, a pair of smaller cages may be inserted anteriorily or posteriorily using minimally invasive techniques. U.S. Pat. No. 6,241,769 to Nicolson et al, et al. discloses a box form cage having a central void having an open top and bottom and a dovetail system for structurally attaching the device to the adjacent vertebra which are prepared by cutting cooperative channels in their surface. Other designs are secured by upper and lower flanges that are rigidly secured to the adjacent bone by screws. The applicant's own U.S. patent application number 12/660,153 filed Feb. 19, 2010 and which is incorporated herein by reference discloses a cage of this design type.
Commonly, supplementary instrumentation in the form of rods (posteriorily) or plates (anteriorily) rigidly fixed to the vertebra are also implanted to stabilize the spine and provide enhanced mechanical stability prior to fusion. These rigid appliances span the intervertebral space to maintain the intervertebral disc height and prevent excessive compression of the two vertebrae, which can lead to a weak fusion or even collapse of the graft. However, they can also lead to stress shielding, in which fusion of the vertebrae to the grafted bone is impeded or prevented entirely because the apparatus prevents adequate contact between the vertebra and the graft. Stress shielding, which occurs when plates or rods carry too large of a portion of the bone's load, refers to the reduction in bone density (osteopenia) as a result of removal of normal stress from the bone by an implant causing atrophy. Resorption of the bone graft can exacerbate this problem. It is known that some subsidence, or settling, between the vertebrae at the graft is advantageous to quickly forming a strong fusion. The subsidence increases bone to bone contact, which enhances bone fusion, as predicted by Wolff's law, by enhancing physiological processes involved in bone remodeling.
It would be therefore an improvement in this art to provide a system and method for securing an interbody fusion cage which overcomes the deficiencies of prior known systems and methods. It is an object of the present invention to provide a dynamic interbody cage anchoring system that prevents cage/graft retropulsion or lateral migration but permit axial loading thereby promoting load sharing between the vertebral column and posterior musculature and prevents stress shielding of graft material by anterior instrumentation or anteriorily fixed interbody instruments. It is a further object of the present invention to provide system and method for securing an interbody fusion cage that is sufficiently robust so as to withstand the forces imposed by normal daily activity on the part of the patient and which is adaptable to a wide variety of cage designs.